During the Harvard-Dartmouth hockey match, Oliver is wearing #7 jersey for Harvard. In the penalty box, he tells Jenny that he is concentrating on how he is going to total the Dartmouth player who had him sent to the box. He points to the Dartmouth player, who at this point has just taken down another Harvard player who is clearly wearing #7.
In the hospital towards the end, Jenny's hair on the left side of the pillow changes position every time the camera angle changes, although she herself presumably has not moved.
During the Cornell/Harvard hockey match, before Oliver enters the penalty box, his hockey jersey is almost spotless. After he sits down and takes off his helmet, a large blood smear appears on his jersey near his shoulder.
When Jenny phones Oliver's dad to decline the birthday invitation, Oliver is wearing a different pair of jeans than he is wearing in the next shot when he runs in the streets looking for her.
When Oliver talks with Phil at the hospital, his hand suddenly appears on Phil's shoulder.
When Oliver says Jenny's father bakes cookies, she corrects him and says he makes pastries. When she meets Oliver's parents, she tells them her father bakes cookies for a living. However, it clear throughout the movie that Jenny often says things, true or not, simply to get a reaction from Oliver. It's likely at least one of those times, she was just trying to get a reaction.
When Oliver is playing in the Harvard-Dartmouth game, the crowd is chanting "Go, Leafs, Go". The chant obviously is dubbed from a National Hockey League game in Toronto's Maple Leaf Gardens.
When Jennifer calls Oliver's father, the number she dials is 1-333-334x (Final number is not in the visible frame.), but the number Oliver told her to dial is 1-338-3434.
When Oliver and Jennifer pull up to his house to meet his parents, the shadow of the camera can be seen on the wheel and hood of the car.
A camera shadow can be seen on the street and car when Oliver and Jenny are talking about naming the baby Bozo.